Regulation of hydraulic turbines



Sept. 5, 1967 KERENSKY 3,339,568

' v REGULATION OF. HYDRAULIC TURBINESA Filed Sept. 16,1965 1 4 Sheets-Sheet 1 GOVERNOR CLOSING SERVO-MOTOR 4 Sheets-Sheet 2 Filed Sept. 16, 1965 GOVERNOR c LLTEG SERVO-MOTOR Sept. 5, 1967 3. KERENSKY 3,339,568

REGULATION OF HYDRAULIC TURBINES Filed Sept. 16, 1965 4 Sheets-Sheet :5

p 5, 1967 G. KERENSKY 3,339,568

REGULATION OF HYDRAULIC TURBINES Filed Sept. 16, 1965 4 Sheets-Sheet 4 United States Patent M 3,339,568 REGULATION OF HYDRAULIC TURBINES Gleb Kerensky, Netherton, England, assignor to The English Electric Company Limited, London, England, a British company Filed Sept. 16, 1965, Ser. No. 487,760 Claims priority, application Great Britain, Feb. 27, 1959, 6,865/59 6 Claims. (Cl. 13725) The present application is a continuation-in-part of my copending application Ser. No. 346,347, filed Feb. 20, 1964, now abandoned, which was divided out of application Ser. No. 7,050, filed Feb. 5, 1960.

The invention relates to the regulation of hydraulic turbines operating under substantial heads and with penstocks of substantial length, for example impulse turbines and low-specific-speed reaction turbines.

A hydraulic turbine of this kind conventionally has a quantity-controlling mechanism, for example a needle valve or an adjustable wicket gate mechanism (also known as shutter mechanism, or adjustable guide vanes) and secondary regulating means, which may for example be a jet deflector in the case of an impulse turbine, or a relief valve in the case of an impulse turbine or a reaction turbine.

. A closing movement of the conventional quantity-controlling mechanism, i.e. a movement in the sense of reducing the flow of water through the turbine, under the action of the turbine governor in response to a reduction in load or increase of rotational speed involves a pressure rise in the end of the penstock adjacent to the said quantity-controlling mechanism. This pressure rise, during the half period of oscillation of the column of water in the pipe line, may even force a greater energy flow past the said quantity-controlling mechanism than before the closing movement. During this half period of oscillation (which may be of several seconds) the governor action is therefore ineffective.

The conventional secondary regulating means (such as the jet deflector of an impulse turbine or the relief valve at the turbine end of the penstock of an impulse or reaction turbine) cannot be made to operate effectively when the governor movement is very small or very slow. This conventional secondary regulating means is designed for a flow of the same order as that which passes through the quantity-controlling mechanism to the turbine. One of the causes of this ineffectiveness is that, for reasons of Water economy, the secondary regulating means must not pass or deflect large quantities of water for long periods and, since it is conventional to operate the secondary regulating means through a collapsible link (e.g. a dashpot), such a collapsible link must have a resonably rapid rate of collapse. However, rapid collapse of the collapsible link, whenever governor movement is slow, results in only a fraction or virtually none of the governor movement being transmitted to the secondary regulating means, thereby rendering the secondary regulating means wholly or partially ineffective. This ineffectiveness of the secondary regulating means with respect to small load or speed changes has hitherto been tolerated (i.e. the resultant speed variations have been accepted) or has been provided against by keeping the pipe lines short, the water velocities low and the inertia constant of the machinery high. In many cases, however, it would be advantageous to abolish the need for such tolerance of speed variation or the use of the countermeasures described.

One object of the present invention is to provide means to reduce pressure fluctuations in the penstock arising from small or slow movements of the governor and quantitycontrolling mechanism, so that improved governing is 3,339,568 Patented Sept. 5, 1967 obtained on small changes of speed or load; that is, under substantially steady running conditions.

One proposal for this purpose is described and claimed in my US. application Ser. No. 397,203 which is a continuation of Ser. No. 7,050 from which the parent of the present application was divided out.

According to the present invention, there is provided a hydraulic turbine comprising a governor which operates a quantity-controlling mechanism, i.e. the needle of an impulse turbine or the shutter mechanism of a reaction turbine, and which also operates through a dashpot device a relief valve connected to the penstock of the turbine in the sense to move the relief valve in the sense of opening when the quantity-controlling mechanism moves in the sense of closing, wherein closing movement of the relief valve takes place under control of the collapsing of the dashpot, and wherein means is provided to prevent or to slow down said collapsing over a small part of the relief valve travel nearest to the fully closed position so that over said part of the relief valve travel small or slow movements of the governor cause opening of'the relief valve.

In order that the invention may be clearly understood a number of embodiments thereof will now be described by way of example with reference to the accompanying drawings, of which:

FIG. 1 is a diagram of a control arrangement for an impulse or reaction turbine,

FIG. 2 is a diagram, similar to FIG. 1, of a contro arrangement for a reaction turbine,

FIG; 3 is a diagrammatic elevation, partly in section, of part of both FIG. 1 and FIG. 2,

FIG. 4 is a modification of the embodiment of FIG. 3, and

FIG. 5 is another modification of the embodiment of FIG. 3.

Referring now firstly to FIG. 1, the control arrangement includes a governor 1 having a servo-motor 11. The piston of the servo-motor 11 operates the needle 2 of an impulse turbine through a linkage 15. The servo-motor also operates a relief valve 23 in the penstock 13, discharging to waste, through a cam 10 and dashpot 16.

Referring to FIG. 2, there is shown an embodiment which is identical to that of FIG. 1 except only that instead of the conventional needle 2, the servo-motor 11 operates a conventional shutter (wicket gate) mechanism 2.

In both FIGS. 1 and 2, the needle 2 (or shutter mechanism 2') is the quantity-controlling mechanism, and the relief valve 23 isthe secondary regulating means.

In conventional operation, rapid changes of load and/ or speed, or of any other parameters to which the governor 1 responds, results in a rapid action by the secondary regulating means which permits eifective regulation by preventing undue pressure rise in the penstock.

For example, the relief valve 23 opens almost as fast as the needle 2 or shutters 2' close, suppressing wholly or partially any pressure rise in the penstock.

However, when the movement of the servo-motor 11 is small or very slow, the conventional secondary regulating means cannot be brought into play effectively.

Here, the Vernier regulation according to the present invention is employed.

Referring now to FIG. 3 of the accompanying drawings, the governor servo-motor 11 is connected to lever 101 so as to move it, in the sense to reduce the turbine speed, in the direction of the arrow. The lever 101 is connected through a rod to the piston 102 of the dashpot device 16, which includes cylinder 103. The latter is the space 106 and the space 107 on the other side being connected to a pressure source, which acts on a greater area in space 106 and thus, when valve 105 is shut, tends to move the servo-piston to the right as seen in FIGURE 3. The servo-piston is connected to the valve member 108 which closes onto a seat 109 and forms part of the valve 23 shown diagrammatically in FIGS. 1 and 2 described above.

When the turbine governor moves in a direction which causes the flow through the turbine to be reduced, it operates a lever or push rod 101 downwards as shown by the arrow and consequently depresses the piston 102 of the dashpot cylinder 103. If the movement is a rapid one there i little lost motion between the piston and the cylinder of the dashpot and the latter therefore depresses the cam 104, which is connected to the valve 105. Opening of the latter releases the pressure on the closing side 106 of the servo-motor controlling the main relief valve 23, which provides the secondary regulating means, enabling the pressure acting on the smaller-area opening side 107 to move the valve shutter 108 off its seat 109, thereby discharging water from the penstock to waste. As the shut: ter 108 moves, it shifts the position of the cam 104 in such a way as to allow the opening of the valve 105 to be reduced by means of a spring; the contour of cam 104 is so designed that the movement of shutter 108 stands in some desired relation to the movement of the governor (e.g. directly proportional to it). If the valve 105 is shut, pressure is re-established on the closing side of the servomotor 106 through the restricted orifice 110, but, if the dashpot 16 were completely rigid, no appreciable reclosure could take place because the smallest movement of the shutter 108 in the closing direction would change the position of earn 104 in such a way as to re-open the distributing valve 105 until an equilibrium wa established between the supply of pressure fluid through restricted orifice 110 and its exhaust through the valve 105 in such a way that the resulting pressure acting on the closing side 106 would exactly balance the force on the opening side 107. Thus re-closure of the main relief valve number 108 takes place only according to the collapsing action of the dashpot 16, which normally occurs as a result of the flow of liquid from one side of piston 102 to the other through some restricted orifice such as that shown at 111 in pipe 112. This orifice is often built into the body of the dashpot piston 102 or cylinder 103 instead of being external to it as shown.

The safety relief valve and non-return valves 124 shown in the body of the dashpot piston 102 are concerned respectively with faulty functioning of the mechanism and with the opening stroke of the governor, and do not concern the invention. When the lever 101 moves upward, corresponding to opening of the quantity-controlling mechanism, the valves 124 open to allow the piston 102 to move upward relative to the cylinder 103.

So far the description is that of the typical function of the main relief valve, used as the secondary regulating means.

The principal reason why turbine relief valves as normally used today do not act as stabilizers during steady running is that, for the sake of economy in water and avoidance of erosion of valve seats, these valves must be given a re-closing action which is relatively rapid, say between seconds and 100 seconds for 100 percent of stroke. This reclosing tendency also operate during the opening stroke, opposing the opening tendency, so that if the governor movement is slow, valve opening is greatly reduced or does not take place at all. Under steady running conditions governor movements are so slow that the relief valve would need a re-closing time of 10 to 100 secs., for only 1 percent of stroke i.e., about 100 times slower than usual, and it is most undesirable to apply that rating to the full stroke as it would mean that every large governor movement would waste a great deal of water and cause great wear on the valve seats. Furthermore an unusual accuracy would be required in the controlling mechanisms of the valve if it were required to make very small but accurate movements.

The present invention is concerned with slowing down the resetting action of the main relief valve 23 when its shutter 108 is in a certain position.

The connecting pipe 112, through which oil can escape from the underside of the dashpot piston 102 to the upper side, is provided with a by-pass pipe through a control valve 113 which completely cuts off the flow through the pipe when the main relief valve shutter 108 is in the closed or near-closed position, thereby entirely precluding any collapsing of the dashpot and thus any reclosing action of the main relief valve; this action may be modified, for example by leakage past valve 113; or by deliberately providing a restricted by-pass through or across the control valve 113, e.g., as represented in FIG. 3 of the accompanying drawings by the restricted orifice 114.

Thus the re-olosing tendency of the relief valve can be reduced to any desired extent for small movements of the governor, but when a large movement occurs the control valve 113 moves beyond the point where it cuts off the flow in pipe 112 and the action of the mechanism thereupon reverts tothat of the normal relief valve.

A by-pass stop-valve 115 is shown, opening of which provides a ready means of making the control valve 113 inoperative when not required. In other words, if the bypass stop-valve 115 is opened the main relief valve 108 ceases to be a vernier stabilizer and reverts to its role as a conventional secondary regulating means.

As explained above, the main obstacle to obtaining accurate response of a turbine main relief valve to the smallest and slowest governor closing movements is the fact that there is a re-closing component in the controls which tends to overcome the opening component. It follows that better opening response can be obtained by reducing the re-closing component as described above. An alternative way of achieving the same object is to prevent collapsing of the dashpot 16 (and consequent closing of the main relief valve 23) during opening movements. This modification will now be described with reference to FIG. 4 of the accompanying drawings, which is a modification of FIG. 3; instead of control valve 113, restricted orifice 114 and stop valve 115, there are now provided a valve 116 in pipe 112, a servo-motor 117 connected to operate the valve 116, an auxiliary dashpot 118 connected to lever 101, a stop valve 119, and a safety valve 120 and a non return valve 121.

Whenever the governor moves in the direction to re duce the flow through the turbine (corresponding to opening movements of the main relief valve) the lever 101 moves downwards and depresses the piston of the auxiliary dashpot 118, thereby forcing some oil into the space on the right-hand-side of the spring-loaded piston of servo-motor 117. This shuts the valve 116, cutting off the flow through the pipe 112 and restricted orifice 111, thus preventing any collapsing of the main dashpot 102, 103.

I As soon as the governor movement stops, leakage past the piston of dashpot 118 reduces the pressure on the servo-motor piston, and allows the spring to move the piston to the right and re-open the control valve 116, thereby allowing the collapsing action of the main dashpot 102, 103 to occur.

A stop-valve 119 is provided for the purpose of making the auxiliary dashpot 118 inoperative when not required, i.e., leaving the control valve 116 permanently open. A safety valve 120 is provided to prevent the building up of excessive pressure in the dashpot 118 when the control valve 116 is on its seat and thus prevents further movement to the left of the piston of servo-motor 117. A non-return valve 121 prevents excessive pressure developing above the piston of dashpot 118 during the opening (upward) movement of the governor lever 101.

There may be cases when it is desirable to have the vernier stabilizer relief valve normally in a part-open rather than the fully-shut position so that it may be eifective to control in both senses of movement. The same can be applied to the main relief valve if used as a Vernier stabilizer, and a practical embodiment of this will now be described with reference to FIG. 5, which is also a modification of FIG. 3 of the accompanying drawings (the parts 111-115 are omited, for the sake of clarity, in this figure).

In this modification the dashpot cylinder 103 is restrained from rising beyond a certain point under the influence of the cam 104 by a lever 122, which itself is partially restrained by spring-loaded stop 123. Therefore, unless an external force is applied to force the dashpot cylinder 103 upward against the restraint of lever 122 and stop 123, the dashpot cylinder 103 stays in a slightly depressed position, and the main relief valve shutter 108 stays partly open. If, however, the dashpot cylinder 103 is pulled upwards by a movement of the governor in the sense of increasing the flow of water through the turbine, the spring-loaded stop 123 will yield and permit further movement of the dashpot cylinder 103 and shutter 108 until the main relief valve is fully shut. When the pull on the dashpot cylinder 103 ceases (because the movement of the governor has ceased), spring-loaded stop 123 acts through the lever 122 and depresses the dashpot cylinder 103, and returns the shutter 108 to its normal part-open position.

To permit the above action non-return valves 124 in the dashpot piston 102 have to be given some sring loading, as shown in FIG. 5, instead of being simple flap valves incapable of offering any resistance to the flow of oil as shown in FIG. 3 of the accompanying drawings, so that when the governor lever 101 moves upward, not only the piston 102 but also the cylinder 103 move correspondingly.

In operation of this modification, when the governor lever 101 moves down, it moves both the dashpot piston 102 and dashpot cylinder 103 solidly, from the normal position which corresponds to the partly-open position of the relief valve 108, and thus moves the relief valve 108 in the sense of opening. The pressure thus exerted on the fluid below piston 102 causes the fluid to leak back slowly to the upper side of the piston, either by leakage past the piston or through pipe 112 and restricted orifice 114, and thus cylinder 103 moves up relative to piston 102, the dashpot slowly collapsing, and the relief valve 108 moves slowly back in the closing direction to its original partopen position.

When the governor lever 101 moves up, it moves both dashpot piston 102 and dashpot cylinder 103 solidly, from the normal position which corresponds to the partly-open position of the relief valve 108, and thus moves the relief valve 108 in the sense of closing. The pressure now exerted on the fluid above piston-102, by reason of the spring acting through stop 123 and lever 122 on the cylin der 103, causes the fluid to leak back slowly to the lower side of the piston, and thus the dashpot slowly collapses and the relief valve 108 moves slowly back, in the sense of opening, to its original part-open position.

It will be appreciated that in this modification, as in FIG. 3, when the relief valve 108 opens substantially beyond its normal position it will open the control valve 113 and the rate of movement of the relief valve will revert to its ordinary (i.e. relatively fast) value.

A catch 125 can be used to lock the lever 122 in a position where it will not interfere with the movement of the dashpot cylinder 103, thereby permitting the main relief valve shutter 108 to return freely to its seat on occasions when it is not desired to run it part-open.

What I claim as my invention and desire to secure by Letters Patent is:

1. A device for the regulation of a water turbine operating under a substantial head of water and having a penstock of substantial length, comprising a speed governor driven by said water turbine; a quantity-controlling mechanism operatively connected to said speed governor and controlling the area available to the mass flow of Water through said penstock to said turbine as a direct function of the control movements of said speed governor; secondary regulating means comprising a relief valve of maximum discharge capacity of the same order of magnitude as the flow to said turbine and connected to said penstock to discharge therefrom; a control element controlling the position of said relief valve; a linkage between said speed governor and said control element, said linkage including dashpot means, said dashpot means including a cylinder and a piston in said cylinder; resilient means loading said dashpot means in the sense to collapse said dashpot means; connecting conduit means between parts of said dashpot cylinder on each side of said piston, whereby to allow collapsing action of the dashpot, which in turn causes resetting of the relief valve inthe other sense after movement thereof in one sense as a result of a control movement of said speed governor; and control valve means in said connecting conduit means, to restrict communication between said parts of said dashpot cylinder, whereby when said control valve means is closed said collapsing action of the dashpot, and consequently the resetting movement of the relief valve, is restricted, whereby the relief valve opens in response to smaller and slower governor movements than when said control valve means is open.

2. A device for the regulating of a water turbine operating under a substantial head of water and having a penstock of substantial length, comprising in combination: a speed governor in driven connection with the said turbine, a primary quantity regulating means operatively connected to said speed governor and controlling the area available to the mass flow of water from said penstock to said turbine as a direct function of the control movements of said speed governor, a pressure regulator of a maximum discharge capacity of the same order of magnitude as the mass flow to the said turbine, a yieldable self-resetting linkage operatively connecting said pressure regulator to the said speed governor in the sense of opening said pressure regulator when the said primary quantity regulating means is moved in the sense of closing, and an additional control member in driven connection with the said pressure regulator, said additional control member reducing the rate of self-reclosure of the said pressure regulator near the fully closed position thereof as compared with its rate of self-reclosure at any position more remote from its fully closed position, so that the divergence between the control movement of said speed governor in the sense of closing said primary quantity regulating means and the opening of said pressure regulator is substantially reduced for small openings of said pressure regulator as compared with large openings thereof.

3. A device for the regulating of a water turbine operating under a substantial head of water and having a penstock of substantial length, comprising in combination: a speed governor in driven connection with the said turbine, a primary quantity regulating means operatively connected to said speed governor and controlling the area available to the mass flow of water from said penstock to said turbine as a direct function of the control movements of said speed governor, a pressure regulator of a maximum discharge capacity of the same order of magnitude as the mass flow to the said turbine, a yieldable self-resetting linkage operatively connecting said pressure regulator to the said speed governor in the sense of opening said pressure regulator when the said primary quantity regulating means is moved in the sense of closing, a dash-pot arranged in the said linkage having a hydraulic communication duct connecting its two ends, an obturator means provided in the said duct and operatively connected to said pressure regulator entirely blocking said duct over a small part of the stroke of said pressure regulator near the fully closed end of stroke of the latter and thereby preventing said dash-pot from reaching a position corresponding to the complete closure of said pressure regulator, and yielding to control movements of the said speed governor as distinct from self-reclosing action without control movements of said speed governor in the sense of then permitting self-reclosing action.

4. A device as claimed in claim 3, wherein the said obturator means has a stroke limiter having one position inhibiting the said obturator from restraining the selfresetting movement of said dash-pot, locking means locking said stroke limiter in its inhibiting position, thereby allowing the said pressure regulator to operate with the self-reclosing action adjusted by its dash-pot.

5. A device for the regulation of a water turbine operating under a substantial head of water and having a penstock of substantial length, comprising a speed governor driven by said water turbine; a quantity-controlling mechanism operatively connected to said speed governor and controlling the area available to the mass flow of water through said penstock to said turbine as a direct function of the control movements of said speed governor; secondary regulating means comprising a relief valve of maximum discharge capacity of the same order of magnitude as the flow to said turbine and connected to said penstock to discharge therefrom; a control element controlling the position of said relief valve; a linkage between said speed governor and said control element, said linkage including dashpot means, said dashpot means including a cylinder and a piston in said cylinder; connecting conduit means between parts of said dashpot cylinder on each side of said piston, whereby to allow collapsing action of the dashpot, which in turn causes resetting of the relief valve in the other sense after movement thereof in one sense as a result of a control movement of said speed governor; and control valve means in said connecting conduit means, to restrict communication between said parts of said dashpot cylinder, whereby when said control valve means is closed said collapsing action of the dashpot, and consequently the resetting movement of the relief valve, is restricted, whereby the relief valve opens in response to smaller and slower governor movements than when said control valve means is open; and comprising also connecting link means between said relief valve and said control valve means, said control valve means being closed so as at least to restrict the flow through said connecting conduit means at least when the relief valve is in the closed position.

6. A device for the regulation of a water turbine operating under a substantial head of water and having a penstock of substantial length, comprising a speed governor driven by said water turbine; a quantity-controlling mechanism operatively connected to said speed governor and controlling the area available to the mass flow of water through said penstock to said turbine as a direct function of the control movements of said speed governor; secondary regulating means comprising a relief valve of maximum discharge capacity of the same order of magnitude as the flow to said turbine and connected to said penstock to discharge therefrom; a control element controlling the position of said relief valve; a linkage between said speed governor and said control element, said linkage including dashpot means, said dashpot means including a cylinder and a piston in said cylinder; connecting conduit means between parts of said dashpot cylinder on each side of said piston, whereby to allow collapsing action of the dashpot, which in turn causes resetting of the relief valve in the other sense after movement thereof in one sense as a result of a control movement of said speed governor; and control valve means in said connecting conduit means, to restrict communication between said parts of said dashpot cylinder, whereby when said control valve means is closed said collapsing action of the dashpot, and consequently the resetting movement of the relief valve, is restricted, whereby the relief valve opens in response to smaller and slower governor movements than when said control valve means is open; and comprising also a servo-motor operatively connected to said control valve means, and auxiliary dashpot means including piston and cylinder elements, one of said elements of said auxiliary dashpot means being connected to said speed governor to move, on movement of said speed governor in one sense, so as to displace fluid in said auxiliary dashpot means, and the cylinder element being connected to said servo-motor, whereby, on movement of said speed governor in said one sense, relative movement of said piston and cylinder elements of said auxiliary dashpot means occurs, displacing fluid into said servo-motor to close said control valve means whereby to restrict collapsing action of the dashpot in the linkage between said speed governor and said control element, and thereby to restrict resetting action of said relief valve on movement of said speed governor in said one sense.

References Cited UNITED STATES PATENTS 853,786 5/1907 Henry 253-24 1,202,310 10/1916 Pfau 13725 1,762,813 6/1930 Foulds i 13725 X 1,769,838 7/1930 Jackson 137-18 X 3,051,186 8/1962 Fisch 13725 CLARENCE R. GORDON, Primary Examiner. 

1. A DEVICE FOR THE REGULATION OF A WATER TURBINE OPERATING UNDER A SUBSTANTIAL HEAD OF WATER AND HAVING A PENSTOCK OF SUBSTANTIAL LENGTH, COMPRISING A SPEED GOVERNOR DRIVEN BY SAID WATER TURBINE; A QUANTITY-CONTROLLING MECHANISM OPERATIVELY CONNECTED TO SAID SPEED GOVERNOR AND CONTROLLING THE AREA AVAILABLE TO THE MASS FLOW OF WATER THROUGH SAID PENSTOCK TO SAID TURBINE AS A DIRECT FUNCTION OF THE CONTROL MOVEMENTS OF SAID SPEED GOVERNOR; SECONDARY REGULATING MEANS COMPRISING A RELIEF VALVE OF MAXIMUM DISCHARGE CAPACITY OF THE SAME ORDER OF MAGNITUDE AS THE FLOW TO SAID TURBINE AND CONNECTED TO SAID PENSTOCK TO DISCHARGE THEREFROM; A CONTROL ELEMENT CONTROLLING THE POSITION OF SAID RELIFE VALVE; A LINKAGE BETWEEN SIAD SPEED GOVERNOR AND SAID CONTROL ELEMENT, SAID LINKAGE INCLUDING DASHPOT MEANS, SAID DASHPOT MEANS INCLUDING A CYLINDER AND A PISTON IN SAID CYLINDER; A RESILIENT MEANS LOADING SAID DASHPOT MEANS IN THE SENSE TO COLLAPSE SAID DASHPOT MEANS; CONNECTING CONDUIT MEANS BETWEEN PARTS OF SAID DASHPOT CYLINDER ON EACH SIDE OF SAID PISTON, WHEREBY TO ALLOW COLLAPSING ACTION OF THE DASHPOT, WHICH IN TURN CAUSES RESETTING OF THE RELIEF VALVE IN THE OTHER SENSE AFTER MOVEMENT THEREOF IN ONE SENSE AS A RESULT OF A CONTROL MOVEMENT OF SAID SPEED GOVERNOR; AND CONTROL VALVE MEANS IN SAID CONNECTING CONDUIT MEANS, TO RESTRICT COMMUNICATION BETWEEN SAID PARTS OF SAID DASHPOT CYLINDER, WHEREBY WHEN SAID CONTROL VALVE MEANS IS CLOSED SAID COLLAPSING ACTION OF THE DASHPOT, AND CONSEQUENTLY THE RESETTING MOVEMENT OF THE RELIEF VALVE, IS RESTRICTED, WHEREBY THE RELIEF VALVE OPENS IN RESPONSE TO SMALLER AND SLOWER GOVERNOR MOVEMENTS THAN WHEN SAID CONTROL VALVE MEANS IS OPEN. 